The present invention relates to a lithium based battery, such as a lithium secondary battery or a lithium ion secondary battery, which is operable with a high level of safety.
A non-aqueous secondary battery is assembled by preparing a cell structure group formed by stacking unit cells each including a positive electrode, a negative electrode, and a separator interposed therebetween, or winding an integral body of the unit cells; containing the cell structure group in a battery container; and filling the battery container with a non-aqueous electrolyte. In addition, the positive electrode is formed by supporting a positive active material such as lithium cobaltate on a collector such as an aluminum foil, and the negative electrode is formed by supporting a negative active material such as graphite on a collector such as a copper foil.
The above nonaqueous electrolyte secondary battery employs a material with its reactivity higher than that of an aqueous electrolyte secondary battery, and therefore, it must be operated with attention given, particularly, to the safety measure thereof. From this viewpoint, for example, a method (1) of providing a safety valve for releasing a high pressure gas from the inside of a battery container, a method (2) of using a PTC device, and a method (3) of using a shutdown separator for limiting a current flowing at the time of outer short-circuit or an inner short-circuit, have been disclosed (see Japanese Patent Laid-open Nos. 2000-58065, 2000-100408, and 2000-133236).
The PTC device in the method (2), which has a PTC (Positive Temperature Coefficient) characteristic, is configured such that the resistance becomes higher with an increase in temperature in the battery, to limit a current flowing at the time of outer short-circuit. The shutdown separator in the method (3) is configured to be melted when heated at a high temperature, to lose the ion impermeability thereof. Accordingly, if the shutdown separator is inserted between electrodes, it is possible to limit a current flowing between the electrodes at the time of outer short-circuit or inner short-circuit.
By the way, if a sharpened metal rod such as a nail pierces a battery as shown in FIG. 16, the metal rod penetrates a positive electrode 1 and a separator 3, and reaches a negative electrode 2. As a result, a positive collector 1a and a positive active material are brought into direct-contact with the metal rod 9 and also a negative collector 2a and a negative active material are brought into direct-contact with the metal rod 9, so that the positive electrode 1 is internally short-circuited with the negative electrode 2 via the metal rod 9. In this case, since a current flows only in the battery, the current limitation by the PTC device in the method (2) is useless, and the shutdown separator in the method (3) also fails to prevent a large current from flowing between the positive and negative electrodes 1 and 2 at the instant when the metal rod 9 pierces the electrodes 1 and 2 via the separator 3.
When a battery is crashed, the separator 3 may be often broken, to cause short-circuit between the positive electrode 1 and the negative electrode 2. In this case, the PTC device in the method (2) is useless, and the shutdown separator in the method (3) also fails to prevent a large current from flowing between the positive and negative electrodes 1 and 2 at the instant when the metal rod 9 pierces the electrodes 1 and 2 via the separator 3.
In this way, the prior art battery is disadvantageous in that if there happens a severe accident due to external causes, for example, if a nail pierces the battery or the battery is crashed, a significantly large short-circuit current instantly flows between electrodes, to bring the battery into a high temperature/high pressure state, with a result there occurs a fear that the battery is ignited and/or burst. The prior art battery, therefore, has a problem that it cannot keep a sufficient safety.